Inﬂuence of surface processes and initial topography on lateral fold growth and fold linkage mode M. Collignon 1 , N. Fernandez 2,3 , and B. J. P. Kaus 2 1 Geological Institute, ETH Zürich, Zürich, Switzerland, 2 Institute Geosciences, Johannes Gutenberg University, Mainz, Germany, 3 Now at Bureau of Economic Geology, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas, USA Abstract Elongation of randomly distributed fold segments and their potential linkage are important for hydrocarbon exploration because it can greatly inﬂuence the morphology of the reservoir and both migration and accumulation of hydrocarbons in antiformal traps. Here we study the effects of surface processes and the presence of a topographic slope on the different linkage modes that can occur, and how these parameters affect the required horizontal offset for perturbations to link. The proposed numerical model represents a sedimentary cover detached over a much weaker basal décollement layer. The upper surface is modiﬁed by mass redistribution, which is achieved by a combination of ﬂuvial and hillslope processes. Several series of simulations were performed: (1) without surface processes or regional slope, (2) with regional slope only, (3) with ﬂuvial incision and hillslope processes, and (4) with hillslope processes only. Model results show that the presence of a regional slope reduces the critical distance required for the transition between linkage and no linkage modes, whereas erosion and redeposition of sediments, on the contrary, increase this distance. The location of the saddle point, where fold segments link, and its vertical distance to the crests of the anticlines are different compared to the case without erosion or initial topographic slope, which potentially can affect the morphology of hydrocarbon traps. Moreover, both erosion and redeposition of sediments enhance the fold elongation (growth along the fold axis), once the erosion velocity exceeds the folding velocity. Model results have been compared to the Zagros Fold Belt. 1. Introduction Many analogue [Abbassi and Mancktelow, 1992; Cobbold, 1975; Mancktelow, 2001], theoretical [Biot, 1961, 1965, 1966; Fletcher, 1991], and numerical [Burg and Podladchikov, 1999, 2000; Schmalholz et al., 2002; Schmid and Podladchikov, 2006] studies have investigated the mechanics of folding instability and fold growth for various rheologies, e.g., viscous and viscoelastic [Biot, 1961], viscous power law [Fletcher, 1974], and viscoelasto plastic [Yamato et al., 2011]. Despite notable analogue [Ghosh and Ramberg, 1968; Grujic, 1993; Johns and Mosher, 1996] and numerical [Fletcher, 1995; Kaus and Schmalholz, 2006; Schmalholz, 2008; Schmid et al., 2008] studies, many aspects of three-dimensional folding, such as fold elongation and fold linkage, are not fully understood. Fold linkage is important for hydrocarbon exploration, as it can greatly inﬂu- ence both migration and accumulation of hydrocarbons in antiformal traps [Jolley et al., 2007; Sales, 1997]. Landscape geomorphology provides an indirect observation of the tectonic activity. Surface processes and tectonics interact to create a wide variety of landscapes. Studies of the drainage network and quantitative geomorphic analyses can be used to understand the history of fault segments [Delcailleau et al., 2006; Vergés, 2007]. Asymmetry of the drainage network, lateral deﬂection of rivers, and/or the presence of one or several wind gaps can record ampliﬁcation and elongation of embryonic fold segments [Bretis et al., 2011; Keller et al., 1999; Ramsey et al., 2008]. These fold segments would lengthen in the direction along fold axis and link together to form long folds. This mechanism has been suggested for the Zagros Fold Belt, both in the Fars Province [Ramsey et al., 2008] and in Kurdistan, NE Iraq [Bretis et al., 2011], where the axial lengths of single folds can reach more than 100km. Following the theoretical model of Bretis et al. [2011], Grasemann and Schmalholz [2012] have numerically investigated the distance between two isolated elongating folds to explain four different modes of linkage: (1) linear linkage, (2) oblique linkage, (3) oblique no linkage, and (4) linear no linkage. However, the effects of surface processes on three-dimensional fold growth were not considered in their work. Interactions between erosion, sedimentation, and mountain building have been extensively studied. In general, while syntectonic sedimentation appears to control the development of basins in the external parts COLLIGNON ET AL. EROSION, SEDIMENTATION, AND FOLD LINKAGE 1 PUBLICATION S Tectonics RESEARCH ARTICLE 10.1002/2015TC003843 Key Points: • Effects of erosion, sedimentation, and topography on fold linkage • Effects of topography on the saddle point location and the reservoir closure • Interaction between small-amplitude solution and erosion velocity Correspondence to: M. Collignon, marine.collignon@erdw.ethz.ch Citation: Collignon, M., N. Fernandez, and B. J. P. Kaus (2015), Inﬂuence of surface processes and initial topography on lateral fold growth and fold linkage mode, Tectonics, 34, doi:10.1002/ 2015TC003843. Received 4 FEB 2015 Accepted 3 JUL 2015 Accepted article online 14 JUL 2015 ©2015. American Geophysical Union. All Rights Reserved.